Description:
------------
Regular expression does not detect a case of first letters in cyrillic strings.

Test script:
---------------
TEST 1: preg_match( '/(?=ял)/ui', 'яЛ', $matches); var_dump($matches);
RESULT 1 IS CORRECT:
array(1) {
  [0]=>
  string(0) ""
}

TEST 2: preg_match( '/(?=ял)/ui', 'Ял', $matches); var_dump($matches);
RESULT 2 IS INCORRECT:
array(0) {
}

RESULT 2 MUST BE:
array(1) {
  [0]=>
  string(0) ""
}

Expected result:
----------------
RESULT 2 MUST BE:
array(1) {
  [0]=>
  string(0) ""
}

Actual result:
--------------
RESULT 2 IS INCORRECT:
array(0) {
}